Field of the Invention
The present invention relates to a protective film detecting method for detecting a water-soluble protective film with which a work surface of a workpiece such as a semiconductor wafer is coated.
Description of the Related Art
As a method for dividing a wafer such as a semiconductor wafer and an optical device wafer along streets, there has been proposed a method in which a laser beam is applied along streets formed on a workpiece such as a wafer to form laser-processed grooves, and the workpiece is broken along the laser-processed grooves by a mechanical breaking apparatus (see, for example, Japanese Patent Laid-open No. Hei10-305420).
When the workpiece is irradiated with the laser beam along the streets in this processing method, thermal energy is concentrated on irradiated regions to generate debris, which may be deposited on the surfaces of devices, leading to a new problem that the quality of the devices is lowered. In order to solve the problem arising from the deposition of debris, a laser processing machine has been proposed in which a work surface of a workpiece is coated with a water-soluble protective film formed of polyvinyl alcohol or the like, and a laser beam is applied to the workpiece (wafer) through the water-soluble protective film (see, for example, Japanese Patent Laid-open No. 2007-201178).
The water-soluble protective film may need to coat those parts of the work surface of the workpiece at which lowering in device quality may be brought about by deposition of debris. In the laser processing machine disclosed in Japanese Patent Laid-open No. 2007-201178, however, a region or regions not coated with the water-soluble protective film may be generated in the work surface of the workpiece, due to such causes as adhesion of a water-soluble liquid resin (for forming the water-soluble protective film) in a nozzle for jetting the liquid resin, mixing of air bubbles into the liquid resin, etc. Since the problem due to debris is generated in the regions having failed to be coated, it is necessary, after the coating step with the water-soluble protective film, to check whether the desired regions have actually been coated with the water-soluble protective film.